This invention relates to polyester resins and paints or chemical coatings made therefrom and, more specifically, it relates to polyester resins useful in the preparation of chemical coatings which can be applied to umprimed surfaces.
Chemical coatings for use as can and coil (coiled metal sheet) coatings must exhibit both good flexibility and hardness and they must be capable of being applied in thin films without surface defects such as cratering, etc. Polyester coatings which meet these requirements are described in U.S. Pat. Nos. 3,714,090 and 3,714,091 issued Jan. 30, 1973 and Jan. 30, 1973, respectively. However, those coatings have to be applied over a primer coating in order to obtain the benefit of their improved properties.
Several different types of coatings are presently available for can and coil coating applications which do not employ a primer. These include acrylic, vinyl and polyester coatings. However, each of these types of coatings has several disadvantages.
The acrylic coatings, even though they are relatively expensive, must be used with high Kauri-Butanol (KB) solvents (KB ratings well in excess of 100) which often results in application problems such as streaking and cratering. Additionally, these coatings have a strong tendency to blister during baking (curing) and the cured coatings with the required flexibility are not as hard as is generally desired.
The vinyl-based coatings are also very expensive. Additionally, they are characterized by relatively poor exterior durability and have rapid dirt pickup.
The one-coat polyester coatings, although substantially less expensive than the vinyl and acrylic coatings, also require high KB solvents with attendant application problems resulting in cratering, etc. They also exhibit incompatibility problems with other materials such as those which might be present in coating tanks, etc., which have not been perfectly cleaned following an earlier run with a different composition.